The Quiet American by Graham Greene

As young Pyle's well-intentioned policies blunder into bloodshed, Fowler, a seasoned and cynical British reporter, finds it impossible to stand safely aside as an observer. But Fowler's motives for intervening are suspect, both to the police and himself, for Pyle has stolen Fowler's beautiful Vietnamese mistress.
Originally published in 1956 and twice adapted to film, The Quiet American remains a terrifiying and prescient portrait of innocence at large.